The present invention relates to a method for controlling a controlled system based on an output signal of a sensor, and more particularly, to a control method based on output signals of a plurality of sensors.
In a data storage device (hereinafter, referred to as a “disk drive”) for recording data in a rotational recording medium such as a hard disk device, means for coping with off-track generated by vibrations is essential. For example, off-track becomes a problem when data recorded on a data-track other than a target data track is overwritten during data write. Therefore, a method exists, in which a sensor for vibration shocks is provided beforehand in the disk drive, and a control for interrupting to write data is performed in the case of detecting vibrations. Moreover, there is a method for controlling a head so as to follow the center of a track by adding a correction signal proportional to a vibration to a servo loop for an actuator of a disk drive in the case where the sensor detects the vibration.
Recent years, a track density has been increased more with an increase in capacity of a recording medium. Thus, the off-track has been apt to occur even by a vibration generated by which the disk drive itself seeks the head or by a vibration generated by a disk drive adjacent to the concerned disk drive in a system such as a disk array. Accordingly, in the above-described control method in which the data write is interrupted at the time of vibration generation, the lowering in performance of the disk drive must be brought about.
Here, as means for coping with the off-track in the hard disk device, a control method will be considered in which a correction signal is added to a servo loop. In the hard disk device, an arm that indicates a head rotates around a rotation shaft by a voice coil motor (VCM), and thus the head seeks for a desired track of the recording medium. Therefore, a vibration for allowing the disk drive to rotate parallel to the surface along which the arm rotates (hereinafter, referred to as a “vibration in the rotational direction”) changes a relative position between the VCM and the recording medium, thus generating the off-track. Accordingly, a sensor for detecting the vibration in the rotational direction is provided in the disk drive. As such a sensor for detecting the vibration in the rotational direction, a rotational acceleration sensor for directly sensing a rotational acceleration or a sensor composed by combining two sensors that are linear acceleration sensors for sensing accelerations in the linear direction is usually used. From a viewpoint of cost, the sensor configured by combining the two linear acceleration sensors is used more often.
FIGS. 10(A) and 10(B) are schematic views showing sensors for detecting these vibrations in the rotational direction: FIG. 10(A) shows the configuration obtained by combining two linear acceleration sensors; and FIG. 10(B) shows the rotational acceleration sensor.
In FIG. 10(A), when the disk drive 1010 vibrates in the linear direction (Lv in the drawing), the sensors 1011 and 1012 sense the same accelerations individually, and therefore, a difference therebetween becomes 0. Meanwhile, when the disk drive 1010 vibrates in the rotational direction (Rv in the drawing), a difference occurs between the accelerations sensed by the sensors 1011 and 1012, and therefore, it is detected that a vibration in the rotational direction is given to the disk drive 1010.
In FIG. 10(B), when the disk drive 1020 vibrates in the rotational direction, a rotational electrode in the sensor 1021 rotates, and thus, an electrostatic capacitance between a fixed electrode and the rotational electrode is changed. Therefore, it is detected that a vibration in the rotational direction is given to the disk drive 1020.
When the vibration in the rotational direction is detected by combining the linear acceleration sensors in such a manner as described above, theoretically, the vibration in the rotational direction is detected accurately as described above. However, actually, owing to variations in gain of the linear acceleration sensors the vibration in the linear direction may sometimes be detected as the vibration in the rotational direction. In other words, when gains of two linear acceleration sensors are not equal to each other, even if a vibration in the linear direction is the same, the accelerations sensed by the two linear acceleration sensors will differ from each other, and a difference therebetween is mistakenly detected as a vibration in the rotational direction. Such variations in gain of the linear acceleration sensors occur in the manufacturing process of the linear acceleration sensors, and it is difficult to remove the variations completely.
Moreover, due to a characteristic vibration based on a flexure of a disk drive's substrate onto which the two linear acceleration sensors are attached or the like, it can occur that a vibration in the linear direction is converted into a vibration in the rotational direction, and that a vibration is not transmitted to the two linear acceleration sensors while keeping a correct level. Also in such cases, a vibration component in the linear direction will be undesirably included in the vibration detected by the sensors.
Furthermore, even if the vibration in the rotational direction of the disk drive is detected by use of the rotational acceleration sensor, the rotational acceleration sensor undesirably will have some sensitivity for the vibration in the linear direction when the center of gravity of the rotational electrode and the center of rotation thereof do not coincide with each other accurately due to an error in the manufacturing process of the rotational acceleration sensor or the like.
When the vibration component in the linear direction is included in the vibration detected by the sensor as described above, a correction based on the vibration in the linear direction, which does not actually affect the off-track, will be undesirably made if a correction signal based on the above vibration detection is added to the servo loop for the actuator of the disk drive. Therefore, such a correction will rather cause the off-track.
Note that the above description has been made by exemplifying the case where the disk drive is a hard disk device and the vibration in the rotational direction is required to be detected. However, it may sometimes be required to detect a signal in a direction other than the rotational direction depending on the type of the disk drive, and it may sometimes be required to detect various vibrations in objects other than the disk drive.